Without limiting the scope of the present invention, its background will be described in relation to forming a window in a casing string for a multilateral well, as an example.
In multilateral wells it is common practice to drill a branch or lateral wellbore extending laterally from an intersection with a main or parent wellbore. Typically, once the casing string is installed in the parent wellbore, a whipstock is positioned in the casing string at the desired intersection and then one or more mills are deflected laterally off of the whipstock to form a window through the casing sidewall.
In certain installations, it is desirable to drill the lateral wellbore out of the high side of the parent wellbore. In such installations, it is necessary to form the window in the high side of the parent casing. One proposed solution is to pre-mill the window in the casing, that is, form the window through the casing sidewall prior to installing the casing in the parent wellbore. The casing is then installed in the wellbore and rotated such that the window is in the desired location and orientation.
It has been found, however, that if the casing is to be cemented in the main wellbore, the window must be closed during the cementing operation, such as by using an internal or external sleeve. Typically, the sleeve is made of an easily milled material or is made so that it can be retrieved after the cementing operation. Although such sleeves have achieved some success, they have problems. For example, the sleeve material may be incompatible with fluids used in the well. The use of an external sleeve increases the casing outer diameter, requiring either a smaller casing size to be used, or a larger wellbore to be drilled. The use of an internal sleeve reduces the casing inner diameter, restricting the passage of fluids and equipment through the casing. The use of a shiftable or retrievable inner sleeve requires another operation in the well and increases the complexity of the equipment and the procedure.
In addition, it has been found, that circumferentially orienting a casing string with a pre-milled window is difficult. Specifically, due to the large diameter, long length, high string weight and the friction between the casing string and the borehole, high torque is required to rotate the casing string. Such rotation of the casing string can cause damage to the casing string or the pre-milled window and may lack the precision necessary to properly orient the pre-milled window to the high side.
Accordingly, a need has arisen for improved systems and methods of constructing a multilateral well that include one or more branch wellbores extending from a main wellbore. In addition, a need has arisen for such improved systems and methods that do not require forming the window through the casing sidewall prior to installing the casing in the parent wellbore. Further, a need has arisen for such improved systems and methods that do not require circumferential orientation of the casing string once it has been run in the parent wellbore.